CA1073708A - Oxidation resistant iron base alloys - Google Patents

Abstract

ABSTRACT OF THE INVENTION

An improved oxidation resistant iron base alloy with reduced heat affected zone cracking is provided consisting essentially of about 0.05 to 0.7% carbon, less than 0.3%
columbium, about 15 to 30% cobalt, about 18 to 25% chromium, about 0 to 2% manganese, about 1 to 6% molybdenum, about 0.10 to 0.30% nitrogen, about 15 to 30% nickel, about 0.2 to 0.6%
silicon, about 0.1 to 2% tantalum, about 1 to 10% tungsten, about 0 to 0.1% zirconium, about 0 to 0.5% aluminum, about 0 to 0.1% lanthanum and the balance iron > 23% with incidental impurities not exceeding 0.6% in aggregate. The combined % of columbium and tantalum being at least 0.4% and the combined carbon and nitrogen being > 0.2%. The compositions are in percent by weight.

Description

1~737~8 This invention relates to oxidation resistant iron ba~e alloys and particularly to such alloys having reduced heat afec-ted zone cracking.
We have discovered that columbium in exce~s of a critical small amount, hereafter described, is deleterious to high oxidation resistance in the iron base alloy of this inven-tion, that tantalum within certain limits promotes high tempera-ture static oxidation resistance, that carbon and nitrogen are interrelated in their effect on mechanical properties, that a minimum aluminum content is necessary to assure optimum oxida-tion resistance, that a small but effective amount of zirconium markedly improves thermal fatigue resistance and that the control of these elements in onjunction with one another produces unique and highly desirable properties in iron base alloys.
Thi9 invention is related to and is an improvement upon Canadian;Patent 981,063 of Robert Herchenroeder. In Canadian Patent 981,063, an alloy with improved weldability and oxidation resistance i5 disclosed. It has now been determined that a particularly useful and novel alloy can be provided through the careful control and regulation of the elements Cb, ~ Ta, Al, C and ~, and Zr in a similar composition.
- In the continuing search for h~gh performance materials which will with~tand adverse environmental conditions such as high temperature and oxidizing atmosphere~, the aspect of cost has become increasingly important~
Costs have been accented because significant portions to the total production of many high performarlce alloys, often called superalloy~, are reduced to scrap during the manufacture of the complex designed parts in which the~e alloy~ are commonly used.
Far too often these ~crap~ of intrinsically valuable ' ~ .

~L~73708 materials, are mixed and become nearly useless because the over- -all composition of the scraps do not permit recycling of the scraps into melts of the parent alloys.
; As a consequence, these scraps are often sold at a small fraction of their intrinsic value to foreign concerns.
This contributes adversely to our nation's balance of payments and to our nation's economic well being.
It is a purpose of this invention to provide a superior high performance alloy which can be produced at relatively low cost because of the utilization of large quantities of mixed alloy scraps.
In the broadest concept the alloy consists essentially in weight percent of: 0.05 - 0.7 C; 15-30 Co: 18-25 Cr, 0-2 Mn; 1.0-6 Mo; 0.10-0.30 N; 15-30 Ni; 0.2-0.8 Si; 0.1-2 Ta; 1-10 W; 0-0.1 Zr, 0-0.5 Al; 0-0.1 La; and a controlled columbium con- -tent not exceeding 0.3%; balance ~ 23 Fe plus incidental èlements for example B, Ti, Mg, Cu, Sr P, V, Ca which should not exceed abou~ 0.6 weight percent in the aggregate. The Cb and Ta should be 0.4 minimum and the C and ~ should be > 0.2.

In a particular embodiment the alloy includes at least one of aluminium in an amount of 0.1 to 0.5%, zirconium in an amount of 0.001 to 0.1% and lanthanum in an amount of Oo OOl to O . 1% . `
A narrower preferred range of composition consists ~`

essentially in weight percent of:

Al O - 0.5 C 0.~5 - 0.16 Cb < 0.20 ' !
Co 15 - 25 Cr 18 - 25 Mn O - 2 Mo 2 - 5 .. ~ .
, . :" ~ : ,, ' . ~ :, (~737~18 0.10 - 0.~5 ,, Ni 15 - 2 5 Si 0.2 - 0.. 5 Ta 0.3 - 2 : Zr 0 - 0.1 La 0 ~ 0.1 Balance ~ 23 Fe plus incidental elements such as B, Ti, Mg, Cu, S, P, V, Ca which should not exceed about 0.6% in the aggregate. The Cb and Ta shauld be > 0.4 and the C and N should be > 0.2.
A more preferred embodiment consists essentially in `~ . weight percent of:
;~ Al - 0.1 - 0.5 -- :~
:
C 0.05 - 0.16 :, Cb < 0.2 ` Co 15 - 25 :~
~, . . ~, ., Cr 19 - 23 Mn 0.5 - 2.0 :
' ,. ~
N 0.10 - 0.25 C + N ? 0.25 : Ni 15 - 25 . ~;
Si 0.2 - 0.
Ta 0.4 - 2 ,j , .

Zr 0.001 - 0.1 -~
.
La 0.001 - 0.1 Balance ~ 23 Fe plus incidental elements such as B, Ti, Mg, Cu, S, P, V, Ca and the like which should be < 0.6% in the aggregate.

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1C~737(~8 .
In another aspect o~ the invention there ~s provided a method of producing an improved oxidation resistant iron base alloy comprising melting iron super alloy scrap and controlling the composition of the molten alloy with the ranges, in percent ; by weight, carbon 0.05 to 0.7/0, cobalt about lS to 3~/0, chromium about 18 to 25%, molybdenum about 1 to 6%, nickel about 15 to 3~/0, tungsten about 1 to l~/o, tantalum about 0.1 to 2%, silicon about 0.2 to 0.~/o, nitrogen about 0.10 to 0.3~/0, manganese about 0 to 2%, aluminum about 0 to 0.5%, zirconium about 0 to 0.1%, lanthanum about 0 to 0.1%, and wherein C + N
:~ is greater than 0.2% and Cb + Ta is at least 0.4%, controlling the columbium content so as not to exceed 0.3%, the balance of the composition comprising more than 23% iron with incidental impurities aggregating less than about 0.6%, and casting the molten alloy of said controlled composition.
In the period between 1946 and 1951 a series of United : States patents including patents 2,432,614, 2,432,61S; 2,432,616, ~ .

2,432,617; 2,432,61~3; 2,432,619; 2,513,467;. 2,513,468, 2,513,469~ : .

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~073~)8 2,513,470; 2,513,471 and ~,513,472 were issued to Fran~s and Binder which describe in a broad sense a complex iron base alloy system. An alloy, Multimet (registered trade mark of Cabot Corporation, sometimes referred to as N-155) based upon Franks and Binder's disclosure has been marketed for more than twenty years and is currently covered by Aeronautical Materials Specifications 5532s, 5376B, 5768E, 5769, 5794A and 5795B and Mil-E-17496s.
Franks and Binder in these patents consistently treated columbium and tantalum as total equivalents and frequently treated columbium and tantalum as equivalents of titanium and vanadium -- probably because all of these elements form relative-ly stable carbides. There was no hint that Fran~s and Binder even considered the possibility that exclusion of columbium, vanadium and titanium except as tramp residuals and the predominant use of tantalum rather than colur~bium would yield a markedly superior alloy. We have found particularly that, while columbi~um is an effective strengthening element, the presence of Cb in excess of about 0.3% significantly reduces the oxidation resistance of this alloy. However, since scrap generally carries Cb and it is costly to remove, we can tolerate up to 0.3% but prefer that it be totally absent where economically possible.
P. M. Winslow and R. A. Craun ("Cb+Ta N-155" Solar Aircraft Company, Metallurgical Report M6-12 50) did investigate the partial substitution of tantalum for columbium in the ~-155 composition, but it appears that they did so to determine if tantalum couLd be "tolerated" as an impurity so that an impure source of columbium could be used in the manufacture of alloy N-155, i.e. FeCbTa which has a Cb to Ta ratio of about 10 to 1~
Winslow-Craun concluded that some Ta could be to:Lerated but again there was no hint that the exclusion of Cb and use of Ta in the alloy was highly beneficial~

~73~7~8 The very fact that Multimet (N-155) has been used for twenty years and has been described by AMS 5532B, which speci-fies simply that colur~biurn plus tantalum must be present within the ~ange of 0.75 - 1.25% ~no differentiation at all or sugges-tion of proportions) proves conclusively that those who have used the alloy and those who have made the alloy considered the two elements equivalent on a weight percent basis in effectiveness in the alloy. secause of the relative abundance of Cb com~ared to Ta, colurnbium content of the commercial products generally exceeded substantially tantalurn content. This indicates colum-biurn has been the preferred element of the two.
In those patents of Franks and Binder where aluminum ` is discussed a minimurn level of 0.5% in the absence of boron is required. Apparently, boron and aluminum were considered sub- `
stitutional. In the alloy of this invention, Al and B are not nterchangeable and ~ O.5% Al is considered excessive.
Franks and Binder consistently included both carbon and nitrogen in their specificationsO Nitrogen was usually re-ferred to as "importantly beneficial", or as an aid to high temperature stability. ~o data or evidence was presented which -would indicate how, or in what manner, nitrogen was beneficial, what aspect of high temperature stability was affected by nitro-gen, or that there is an interrelation between carbon and nitro-gen and a critical cornbined amount of nitlogen plus carbon ;; necessary with regard to tensile properties, stress rupture properties, thermal fatigue resistance and weldability. In short, Franks and Binder added nothing to the available know-ledge regarding nitrogen,carbon and their effects upon alloys of the type discussed.
Finally, Franks and Binder do not mention zirconium and the beneficial effects of it on the therrnal atigue resis-tance of the alloy system being discussed.

3L0737(J~
Wlodek (United States Patents 3,3~3,206, 3,304,176 and

3,304,177) discusses a nickel base alloy which contained lantha-num to improve oxidation resistance, but his system was totally different than the alloy of this invention. For example, Wlodek's alloys contained by weight percent 20 Fe max, 6 Co ~ Mn maximum, 8 Mo minimum and a preferred lanthanum content of 0.17, no re-quirements on Al, Ta, Cb or zirconium. The alloy of this inven-tion contains 23 Fe minimum, 15 Co minimum, 6 Mo maximum and requirements on Al, Ta Cb and zirconium.
Wlodek in Patent 3,304,176 specifically shows that cerium and lanthanum are not interchangeable.
Hessenbruch (United States Patents 2,075,718; 2,104,836 and 2,067,569) speaks of cerium and misch metal additions to alloys for heating elements. Hessenbruch's alloys are totally different than the alloy of this invention. Hessenbruch used principally Ce notlanthanum, the base composition d~ffered and he did not claim criticality for Cb, Ta, AL, C and ~ or zirconium.
Thus it is shown that none of the known prior art des-cribes this invention either in composition or concept.
It is believed that a minimum of about 36% nickel plus cobalt is desired to obtain the optimum in oxidation resistance as is 18% Cr - at the lowest possible cost.
The higher levels of Cr, Ni and Co are employed to achieve the better oxidation resistance especially at higher temperatures.
Manganese is an effective spinel constituent and there-fore is included in the preferred embodiments within the range of 0.5 to 2.0%.
Both Mo and W are incorporated in the alloy as solid solution strengtheners and carbide formers to provide needed strength, but the maximum Mo which can be tolerated is less than that of W because of its lower atomic weight for a given weight . .

~7371~3 percentage it raises the average electron vacancy concentration (Nv) of the alloy and promokes the formation of undesirable topologically close packed phases which normally cause embrittle-ment. ~ is undesirable about 10% because of its high density, cost, and degradation of oxidation resistance at very high temperatures.
Silicon as noted in the aforemsntioned Canadian Patent 981,063 is necessary at a minimum level of 0.2% to obtain the optimum oxidation resistance. At levels greater than about 0.5%
silicon tends to promote intergranular oxidation attack and is also detrimental with respect to metallurgical stability.
Chemical analyses of zirconium at the levels noted iA
; this application are probably no more accurate than + 0.005 weight percent. Hence, the prescribed Zr range of 0.001 - 0.1 percent is admittedly somewhat ambiguous but the effects of zirconium as noted are real. Zirconium above about 0.1% is not desired in most products because it tends to widen the liquidus-solidus range of the alloy.
A minimum carbon level of about 0.05% is needed if adequate strength i9 to be obtained.
In wrought products the level of carbon should be less than 0.16 and preferably less than 0.15% if adequate post aged `
ductility is to be maintained. However, if the alloy is to be cast carbon content can be as high as about 0.7%.
The effects of La have been well demonstrated both in aforementioned Canadian Patent 981,063 and in this specification. However, it has been demonstrated by thsse examples that an alloy without lanthanum fa,r superior to the closest current commercial alloy has been discovered. Therefore ; 30 in the broadest embodiment of this invention, lanthanum can be - considered as optional. To achieve the optimum in properties ~ lanthanum must be present in a small but effective amount as noted.
.

',~ ' ' , 73701~3 Lanthanum when used may be adde~ in a variety of forms such as alloyed with Ni, Co, Si or other elements or even in an impure form in conjunction with other "rare earths" such as cerium for economic purpose~. However, the lanthanum content of the addition mixture should substantiaLly exceed the total of the other rare earths present. Lanthanum, cerium and other rare ; earths are not equivalent as noted by Wlodek and others. Excess quantities contribute to "dirt" in the alloy, poor hot work-ability and poor weldability.
Other incidental elements such as those noted are frequently present in alloys of this type either as intentional additions, for example s to achieve higher moderate temperature strength, or simply because they are tramp elements in the raw materials and scrap used to formulate the alloy. In this inven-tion, these elements are preferably maintained at a level less than 0.6% in the aggregate.
The superior quality and unpredicted characteristics ; of the invention can perhaps best be understood by re~erence to the following examples.
Chemical analyses of the alloys used to define this invention are listed in Table I.
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~0737(~

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l 0 0 0 tS~ o o o (O a~ ~ ~n (O (~) ~ (O ~ (~ ~) a~ 6 N N N N --i N N d~ N ~O d' d' o d~ o 0 0 O ~9 -: Q i ~ ~ I ~ ` N d' ~
O O O O O O O O O ~ -1 0 0 0 0 0 0 0 0 .~ V V V V V V V
-' -i 0 ~ ~ N ~i 11 d~ -i N ~ d' d' ~ (~1 N ~
O O o o o o o o o o o o o O o o o o o o O O O ~ O O O O -i ml o o o O O O O O O ' l l ~
V V V V V V V V
~ r` ~D r` ~ w ~ N ~ ~ co d' t~ -1 co d' Z ¦ ~ '-~ '-~ O -i o N r-l N O O O
O O Q O O O O O O O O O O V
. , O ~-- 0 ~) O ';t' -i N 0 d' ~D ~) r~ oo ~ o -1 N d' Ul -i d1 d' d' U l o U~ 0 d' 0 0 0 d' d' d' d' d' d' , 1~737~)8 All of the alloys were initially induction-melted in air in nominally 70-lb. heats using commercial grade raw materials.
Alloys A through K were cast lnto 3-inch diameter elec-trodes and subsequently electro-slag remelted.
Alloys L through T were not remelted but were melted in groups of threeO Nominally, 70-lb. heats of the base alloys L, 0 and R were melted.
After casting the first 20-lb. ingot, a late addition of columbium was made to form alloy M which was cast, and an additional late addition of columbium was made to form alloy N.
In similar manner, alloys P and Q were produced by adding columbium to the base melt of alloy 0, and alloys S and T
were produced by adding columbium to the base melt of alloy R.
In the case of alloys R, S and T, individual late additions of lanthanum were also made.
Alloys A through I were processed simultaneously as were alloys L through T and alloys ~ and K. Forging temperatures were 2050-2150F., hot rolling temperature was 2050F. Portions ' of the alloys were annealed at 2050F. and por~ions at 2150F.
to evaluate their variable. Alloys A through I were cold rolled 20% to improve surface finish and reannealed; alloys J through T
were tested in the as hot rolled, annealed and pickled condition.
All of the alloys had excellent hot and cold work-ability.
Alloy U was a randomly chosen heat of commercially pro-duced material which met the requirements of AMS 5532B.
The procedure for dynamic oxidation tests was a follows:
1. Prepare specimens about 1/16 x 3/8 x 3 inches.
2. Grind all surfaces to a 120 yrit finish and degrease in a solvent such as acetone.
3. Measure e~act surface area and weight of each specimen.

1~737C~8 ; 4. Expose specimens in a holder rotating at 30 RPM to the combustion products of an oil fired fla~e plus excess air moving at a velocity of about 0.3 Mach. ~ ~ -5. Cool to near ambient temperature each 30 minutes. ~ -6. Weigh each sample after every 25-hours of the test for the duration of the tests.
7. Section each sample at a point 2-inches from the base, mount for metallographic examination and optically measure depth of continuous penetration, depth of internal oxidation and unaffected thickness.
8. Calculate average weight loss (mg/cm2).
9. Calculate total depth of affected metal.
The procedure for these static oxidation resistance tests was as follows:
1. Prepare specimens about 3/4" x 3/4" in size and having a thickness of between 0.03 and 0.25".
2. Grind all surfaces to a 120 grit finish and degrease ~ ~, in acetone.
, 3. Measure exact surface area and weight of each specimen.

4. Expose specimens to dry air flow of more than 2 `~
cu.ft./hr. per in.2 of furnace cross section through ` the furnace while maintaining a constant temperature therein for four 25-hour periods with the specimens being air cooled to room temperature after each 25-hour period.

5. Reweigh each specimen.

6. Descale specimens in salt bath.

7. Carefully weigh the descaled specimens and calculate the weight loss of each.

8. Convert these weight loss figures to "average depth of metal lost" values in accordance with the following -~73'70~

: formula:
Measured Weiqht Loss x Density of Alloy Surface Area of Specimen Dynamic oxidation data are presented in Table II and static oxidation data are present in Table III.
TAsLE I I
2000F. DYNAMIC OXIDATION DATA
(lOO hr. test) Weiqht Loss Ma/cm2 ~ 148 16 C 149 5 . .
D 150 15 :

M 138 111 .
. . N 139 260 ; P 141 105 U 5533 300 :
Test results from alloys A - I and the first value listed for alloy U are averages of 4 determinations :Erom 3 tests.
Test results from alloys L - T and the second value for U are single determinations ~rom a single test~

- 12 - .

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: ~`` 3.1373708 ;-T~sL~
2000F. STArrIC OXIDATIO~ DATA
Alloy Descaled Metal Loss, mils/side B 148 0.50 C 14g 0.49 D 150 0.42 F 151 0.57 G 152 0.
H 153 0.77 I 154 0.89 ~-L 137 1.10 M 138 1.40 . : ~
N 139 2.9 ~ ~ -~ 0 140 0.71 - P 141 1.00 ., :
, Q 142 1.40 ~ ~
- . ~

` - S 145 0.59 ~ -` 20 T 146 0.44 ~ ~
, . ~ . .
U 5533 2.12 Alloys L, M, N represent a base alloy with increasing columbium content of 0, 0.24 and 0.70 weight percent respectively, but with no tantalum or lanthanum. It is readily apparent that weight loss because of oxidation in a dynamic environment in-creased as columbium content increased.
Alloys 0, P and Q represent a base alloy containing ~` nominally 1 w/o Ta, no lanthanum, and increasing amounts of columbium of 0.24, 0.40 and 0.60 w/o respectively. It is obvious that Cb is very detrimental to the dynamic oxidation resistance of ~he alloy system.
Note that alloys 0, P and Q which contained Ta had . ~ .
- 13 ~

.... . .

737~

lower metal loss because of oxidation than did alloys L, M and N which did not.
T~sLE IV
AlloyAl Content w/o Weiqht loss mq/cm2 A 147 0.03 27 F 151 0.06 21 ; D 150 G.07 15 s 148 0.07 16 G 152 0.11 7 C 149 0.16 5 Alloys R, S and T represent a base series of alloys containlng nominaLly 1.30 w/o Ta, nominally 0.04-0.06 La and columbium contents of 0.18, 0.30 and 0.26 respectively. Again, the effect of columbium is noted but the effects are dampened by the presence of the small amount of lanthanum.
Alloys A - D, F and G are alloys containing nominally one half percent of Ta, a small but very effective amount of lanthanum and essentially no columbium. The oxidation weiyht losses of these alloys should be compared first to those of alloys H and I and finally to the weight losses measured for alloy U. The results show conclusively that columbium is ex-- tremely detrimental, that tantalum is not and that lanthanum promotes the oxidation resistance of the system.
Two additional effects are to be noted from the data ;
from the severe dynamic oxidation test. First is the effect of `-a small but effective amount of zirconium, in this case 0.01 w/o, on the oxidation resistance. Alloys H and I are alike with the exception that a small addition of zirconium was added to alloy H just prior to casting; none was added to alloy I. This small amount caused a 27% reduction in oxidation loss.

Secondly, to achieve the optimum in oxidation resis-tance, aluminum should be present in the alloy. Table IV com-'.~: ' ' ~,' ' . ' ' ' ~L~737~8 pares the oxidation resistance of the similar alloys A - D, F
and G. The correlation is unmistakable.
Alloy A has excellent cynamic oxidation resistance when compared to the commercial product alloy U, but to optimize this resistance to the ~ullest extent, the alloy should contain at least about 0.1 Al and a small but effective amount of zirconium.
The effects of columbium, tantalum and zirconium on static oxidation resistance can also be noted in the data of Table III. The beneficial effects of Al noted in the dynamic oxidation tests is not readily apparent. -The marked effect of a very small amount of zirconium was also noted in the thermal fatigue resistance of sheet pro-; duct. Also, a dramatic effect of carbon plus nitrogen was noted.
Alloys A - I, which were annealed at 2150F. and U were tested as follows:
1. Sheet samples nominally 1/16 inch thick and 3 inch square were prepared by pac~ grinding the edges of the sheets to be tested so that the resulting grind marks ran parallel to the edges of the sheet and so that the effects of grinding would be uniform.
2. The specimens were rnounted on a rotating drum so that in one group the edges of the specimen heated were parallel to the previous rolling direction and in the second test group the ecges to be heated were perpendicular to the rolling direction.
3. The drum was then rotated at a speed of about 0.3 RPM so that the edges of the specimens passed through a neutral oxyacetylene flame emanating :.
from a No. 72 tip size with about a 6" outer cone, causing a semi-circular heated zone on each specimen.

~73~08 4. ~he maxirnum -temperature of 1650F. was monitored by using a fine wire thermocouple attached to a dummy speclmen .
5. Specimens were evaluated on the basis of thermal cycle for first crack initiation and by total crack length in mils.

The thermal fatigue data are set out in Table V below:

TABLE V

THERMAL FATIGUE RESISTANCE AT 1650F.
Edges Parallel* to Edges Perpendicular to Roll_Direction_ Roll Direction ***

Thermal Total Thermal Total Cycle for Crack Cycle for Crack Crack Length Crack Length C + N -Alloy Initiation Mils. Initiation Mils. %

A 147 118 192 246 150 0.15 B 148 133 152 322 95 0.21 C 149 250 79 406 73 0.37 E 104 150 201 194 223 0.26 i~

F 151 168 176 220 201 0.25 ~ , G 152 245 81 369 95 0.33 H 153 ** ** 406 77 0.31 -~
: :.
I 154 150 219 266 130 0.33 * total thermal cycle 250 ** no cracks *** total thermal cycles 406 ~
Since alloys A, B and C have essentially the same com- ~ -position with the exception of carbon plus nitrogen, these alloys can be compared directly. Alloys E, F and G likewise can be compared in this respect. Furthermore, alloys H and I can be compared. However, the m~mbers of each of the three groups should only be compared within the group because alloys A, B and C have less Ni + Co than do alloys E, F and G and alloys H and I
contain columbium instead of Ta.
Comparing the data of alloys A, B and C and E, F and 1(~737~8 G separately, one can readily see the increase in fatigue resis-tance with the respective increase of C + N content, both in terms of crack initiatlon and total crack lengths.
The most surprising development o all was the dis-covery that alloy H had outstanding thermal fatigue resistance compared to its counterpart alloy I. This unexpected improve-ment is attributed to the small but effective amount of zirconium which was added to alloy H.
The invention is further illustrated by reference to the drawings in which:
Figure 1 is a plot of the average life in hours of the specimens of Table VI versus C, N and C + ~;
Figure 2 is a plot of 0. 2% offset yield strength versus C, N and C + N, Figure 3 is a plot of ultimate strength versus C, N and C + N, Figure 4 is a plot of ultimate strength versus - C, N and C + N, and Figure 5 is a plot of the average life in hours of the specimens of Table Vl versus C, N and C + N.
Data from stress rupture tests at 1500F. 18KSI of alloys A through C and E through I are licted in Table VI, and the average life of the specimens versus C, N and C + N are plotted in Figure 1. For reference, the qualification stress rupture condition of alloys meeting the AMS 5533 B specification is 1500F. - 18KSI - 24 hour life. A11 of these alloys surpass this requirement.

~ 17 -:: .

~L~737~1~

TABLE VI
_ Effects of Carbon and Nitroyen On Averaqe Stress Rupture Life at 1500F.-18KSI

Life Elongation C -~ N
y Hrs. % w/o A 147 31.3 46 0.14 B 148 76,7 `61 0,21 56.6 56 C l~9 91,1 2~ 0,37 85.8 3 E 104 46.7 34 0.26 F 151 177.3 36~ 0.25 G 152 111.7 39 0.33 105.8 42 H 153 199.8 42 0.31 240~5 41 I 154 72.2 52 0.33 128.6 46 When the average stress rupture lives of these alloys are plotted versus either C or N there is no apparent correlation.
However, when the average lives of the specimens are plotted versus combined C plus ~ definite trends appear. There is a valid correlation between C plus ~ content and stress rupture life. Admittedly, two curves are developed but both indicate stress rupture life increases with increased C + ~ and the desira~ility of maintaining the level of C ~ ~ greater than 0.20 and preferably greater than 0.25 is obvious. The reason for the two curves is not understood at present. Possibly a strengthen-; ing precipitate of some sort caused the difference. Electron microscopy did reveal extremely fine precipitates in some of thesamples.
Ultimate tensile and 0.2% offset yield strengths at 1200 and 1600F. show similar correlakions. Tensile data from test performed per ASTM standards at RT, 1200 and 1600F are listed in Table VII and are plotted in E'igures 2 through 5 inclusive.

. .
; ,, ~737~3 TABLE VII
~ TENSILE DATA
; Annealed Sheet TEMPERATURE 0,2% OFFSET ULTIMATE ELO~GATIO~
ALLOY _FYS, KSI KSI %

0.02C
0 13N 120020,6 64,3 68 0 15C+~ -- 19,6 63,8 65 160017,8 37,7 42 ~~ 18,7 37,2 43 0.08C
0 13N 120023,4 68.6 62 0 21C+N -- 24.2 72.4 66 160021.8 ~0.0 44 --- 22,6 41,4 35 0 l9C 120039,1 92.7 49 200 18N -- 38,4 93.6 53 0,37N+C
160027,2 44.4 36 -- 27.7 46,4 46 0.17C
0 09N 120029,4 82.8 65 0 26C+~ -- 29,4 79.1 56 160025,1 43,6 33 - -- 21,6 40,7 36 0,11C 120028,5 78,9 69 0,14N -- 28.7 78,9 69 0.25C+~
160026.3 45.3 32 -- '26.9 47.7 42 0.11C 120038.8 90.9 51 0.22N -- 39.1 91.3 69 0.33C~N
160030.9 4~3.1 40 -- 30.7 48,1 33 ':
: 19 .

7371~

TABLE VII (Continued) ;
TEMPER~TURE O,2% OFFSET ULTIMATE ELONGATION
ALLOY F YS, KSI KSI %
~ 153 0.15C 1200 38,2 88,4 ~7 0,16N -- 38.0 87,0 44 0.31C~
1600 27.1 4301 40 __ 24.5 ~2.2 34 0.15c 0.18N 1200 36.6 87.7 41 0.33C~N -- 37.6 89.6 45 1600 29.3 46.3 25 -~ 29.7 47,0 33 As in the case of the stress rupture data, when tensile or yield strengths are plotted versus either C or N no apparent correlation exists. However, when the data are plotted versus C + ~ combined, very good correlations are established and desirability of controlling the combined total of C and ~ is apparent. Duplex curves as in the stress rupture data are developed at 1600F., and again the causs of this is not com-pletely understood, there is undoubtedly another mechanism operative which adds a significant cumulative effect to that of C plus ~. The fact that the effect is noted at 1600F but not at 1200F where only a single curve is developed further sug- -gests a precipitation phenomena. Thus, with proper heat treat-ments one would anticipate controlling the reaction.
~itrogen is beneficial in reducing heat affect~d zone 30 cracking (HAZ), contrary to accepted teaching. It is yenerally accepted that higher levels of gases will reduce weldability.
Tig-a-ma-jlg tests were performed on pairs of the alloys with similar carbon contents but varying nitrogen content. Table VIII
below illustrates this effect. ;;

, ~L~7370~

TABLE VIII
Weldability ,¦ ::
Average Total HAZ N2 C
AlloyCrack lenqth! Mils % %
E 169 0.09 0.17 C 83 0.18 0.19 F 69 0.14 O.lI
G 12 0.22 0.12 The data ~how that for a given carbon content, increased nitrogen reduced HAZ, and that high carbon content promotes HA2 cracking. (Another basis for a carbon limitation of about 0.15% in the pre~erre~ embodiment.) ;
The data presented and the discussion khereof clearly illustrate that an alloy with heretofore unknown or suspected : qualities has been discovered.
All compositions described are in percent by ~ : .
~:~ weight unless otherwise specifically stated.
~:. - While we have illustra~ed and descri~ed certain pre- 20 ferred embodiments of our invention, it will be under3tood that this invention may be otherwise embodied within the scope of the following claim~. ~
~;

, .~ :

j - 21 -.,

Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An improved oxidation resistant iron base alloy consisting essentially of in percent by weight about 0.05 to 0.7% carbon, about 15 to 30% cobalt, about 18 to 25% chromium, about 0 to 2% manganese, about 1 to 6% molybdenum, about 0.10 to 0.30% nitrogen, about 15 to 30% nickel, about 0.2 to 0.8% silicon, about 0.1 to 2% tantalum, about 1 to l0%
tungsten, about 0 to 0.1% zirconium, about 0 to 0.5%
aluminum,about 0 to 0.1% lanthanum, provided that there is present at least one of aluminium in an amount of 0.1 to 0.5%, zirconium in an amount of 0.001 to 0.1% and lanthanum in an amount of 0.001 to 0.1%; and a controlled columbium content not exceeding 0.3%, wherein C + N is greater than 0.2% and Cb + Ta is at least 0.4%, and the balance > 23% iron with incidental impurities aggregating less than about 0.6%.

2. The alloy of claim 1, containing zirconium in the range 0.001 to 0.1%.

3. The alloy of claim 1, wherein the manganese content is in the range 0.5 to 2%.

4. The alloy of claim 1, containing lanthanum in the range 0.001 to 0.1%.

5. The alloy of claim 1, containing aluminum in the range 0.1 to 0.5%.

6. The alloy of claim 1, having 0.001 to 0.03%
zirconium, 0.5 to 2% manganese, 0.001 to 0.1% lanthanum, and 0.1 to 0.5% aluminum.

7. An improved oxidation resistant iron base alloy consisting essentially of in percent by weight about 0 to 0.5% aluminum, about 0.05 to 0.16% carbon, about 15 to 25%
cobalt, about 18 to 25% chromium, about 0.to 2% manganese, about 2 -to 5%-molybdenum, about 0.10 to 0.25% nitrogen, about 15 to 25% nickel, about 0.2 to 0.5% silicon, about 0.3 to 2%
tantalum, about 1 to 8% tungsten, about 0 to 0 to o.1% zirconium, about o% to 0.1% lanthanum, provided that there is present at least one of aluminium in an amount of 0.1 to 0.5%, zirconium in an amount of 0.001 to 0.1% and lanthanum in an amount of 0.001 to 0.1%: and a controlled columbium content not exceeding 0.2%, wherein C + N is greater than 0.2% and Cb +
Ta is at least 0.4%, and the balance > 23% iron with incidental impurities aggregating less than about 0.6%.

8. The alloy of claim 7,having about 0.1 to 0,5%
aluminum, about 0.5 to 2% manganese, about 0.00l to 0.1%
zirconium and about 0.001 to 0.1% lanthanum.

9. The alloy of claim 7, wherein the aggregate % of carbon plus nitrogen >0.25.

10. The alloy of claim 8, wherein the aggregate % of carbon plus nitrogen >0.25.

11. An improved oxidation resistant iron base alloy consisting essentially of in percent by weight about 0.1 to 0.5% aluminum, about 0.05 to 0.16% carbon, about 15 to 25% cobalt, about 19 to 23% chromium, about 0.5 to 2.%
manganese, about 1 to 6% molybdenum, about 0.10 to 0.25%
nitrogen, about 15 to 25% nickel, about 0.2 to 0.5% silicon, about 0.4 to 2% tantalum, about 1 to 8% tungsten, about 0.00l to 0.1% zirconium, about 0.001 to 0.1% lanthanum, a controlled columbium content not exceeding 0.2%, and the balance >23% iron with incidental impurities aggregating less than about 0.6%
and wherein the aggregate % of carbon plus nitrogen is >0.25.

12. The alloy of claim 11, having about 1 to 4% tungsten and columbium <0.1%.

13. The alloy of claim 11, free of columbium.

14. The alloy of claim 12, free of columbium.

15. The alloy of claim 11, containing columbium in a controlled amount up to 0.2%.

16. A method of producing an improved oxidation resistant iron base alloy comprising melting iron super alloy scrap and controlling the composition of the molten alloy within the ranges, in per-cent by weight, carbon 0.05 to 0.7%, cobalt about 15 to 30%, chromium about 18 to 25%, molybdenum about 1 to 6%, nickel about 15 to 30%, tungsten about 1 to 10%, tantalum about 0.1 to 2%, silicon about 0.2 to 0.30%, nitrogen about 0.10 to 0.30%, manganese about 0 to 2%, aluminum about 0 to 0.5%, zirconium about 0 to 0,1%, lanthanum about 0 to 0.1%, and wherein C + N is greater than 0.2% and Cb + Ta is at least 0.4%, controlling the columbium content so as not to exceed 0.3%, the balance of the composition comprising more than 23%
iron with incidental impurities aggregating less than about 0.6%, and casting the molten alloy of said controlled com-position.

17. A method according to claim 16, wherein in said molten alloy the zirconium content is controlled within the range of 0.001 to 0.1%.

18. A method according to claim 17, wherein in said molten alloy the lanthanum content is controlled within the range of 0.001 to 0.1.

19. A method according to claim 16, 17 or 18, wherein in said molten alloy the manganese content is controlled within the range of 0.5 to 2% and the aluminum content is controlled within the range of 0.1 to 0.5%.

20. A method according to claim 16, wherein in said molten alloy the contents of zirconium, manganese, lanthanum and aluminum are controlled within the following ranges -zirconium 0.001 to 0.03%, manganese 0.5 to 2%, lanthanum 0.001 to 0.1%, aluminum 0.1 to 0.5%.

21, A method according to claim 16, 17 or 18, wherein said molten alloy includes columbium and said columbium content is controlled so as not to exceed 0.2%. 26